The present invention relates to an electrophotographic toner and more particularly to an electrophotographic toner to be used for image forming with the use of an electrostatic copying apparatus, a laser beam printer or the like.
In a magnetic-brush developing method using a two-component developer containing a toner and a carrier, an image is formed according to the following steps in which:
(a) A developer containing an electrophotographic toner is first held around the outer periphery of a developing sleeve incorporating magnetic polarities, thereby to form a so-called magnetic brush; PA1 (b) The magnetic brush is allowed to come in contact with a photoreceptor on the surface of which an electrostatic latent image is being formed, so that the electrophotographic toner is electrostatically stuck to the electrostatic latent image. This causes the electrostatic latent image to be turned into a toner image; and PA1 (c) The toner image is transferred to paper from the surface of the photoreceptor and fixed on the paper by heating-fixing. Thus, image forming is completed. PA1 (1) the drop starting temperature of storage elastic modulus is in the range from 100.degree. to 110.degree. C.; PA1 (2) the storage elastic modulus at 150.degree. C. is not greater than 1.times.10.sup.4 dyn/cm.sup.2 ; and PA1 (3) the peak temperature of loss elastic modulus is not less than 125.degree. C.
As the electrophotographic toner used for the image forming above-mentioned, there may be used an electrophotographic toner as obtained by blending a fixing resin with a coloring agent such as carbon black, a charge controlling agent and the like and by pulverizing the blended body into particles having sizes in a predetermined range.
The electrophotographic toner above-mentioned is required to have (i) off-set resisting properties for preventing the occurrence of so-called off-set such as contamination of the fixing rollers due to partial sticking of a molten toner to the heating rollers, and (ii) fixing properties for preventing the toner image from being defectively fixed on paper when the fixing temperature is low (deterioration of low-temperature fixing properties).
In an electrophotographic toner using a fixing resin having a high molecular weight to satisfy the off-set resisting properties, it is required to set the fixing temperature to a high temperature. This is not preferable from an energy economy point of view. On the other hand, an electrophotographic toner using a fixing resin having a low molecular weight to satisfy the low-temperature fixing properties is poor in heat resistance because the toner particles are agglomerated and solidified to provoke blocking when the inside of the image forming apparatus is heated to a high temperature.
To give both off-set resisting properties and heat resistance to the toner, there have been proposed various examples of an electrophotographic toner jointly containing resin having low molecular weight and resin having high molecular weight (See, for example, Japanese Patent Unexamined Applications No. 16144/1981 and No. 3644/1985).
In a joint use of low-molecular-weight resin and high-molecular-weight resin, it is difficult to properly determine the blending proportion of both resins. If the amount of the low-molecular-weight component is too little, the resulting toner is poor in low-temperature fixing properties. If the amount of the low-molecular-weight component is too much, the resulting toner is poor in off-set resisting properties. In fact, there has not been obtained a toner which simultaneously satisfies both requirements of fixing properties and off-set resisting properties. Further, when a low-molecular-weight resin and a high-molecular-weight resin are merely jointly used, the resulting toner is insufficient in heat resistance.
To improve a toner in these characteristics, there has been offered a proposal in which attention has been placed on toner rheology characteristics. U.S. Pat. No. 4,913,991 discloses a color toner excellent in luster by determining the range of tangent loss (tan .delta.) which represents the ratio of storage elastic modulus to loss elastic modulus. Further, EP-A-407083 discloses a toner excellent in fixing properties and off-set resisting properties by determining the range of the tangent loss (tan .delta.) in a predetermined storage elastic modulus.
However, even the toners having predetermined rheology characteristics set forth in the documents above-mentioned cannot simultaneously satisfy all the requirements of low-temperature fixing properties, off-set resisting properties and heat resistance. More specifically, the toner having rheology characteristics set forth in U.S. Pat. No. 4,913,991 is developed for full-color and therefore made soft such that the toner is readily molten. Accordingly, when the toner is used for mono-color, the toner may readily provoke off-set. The toner having rheology characteristics set forth in EP-A-407083 is not sufficient in heat resistance.